RT ticket 62918: _modpow(x,y,z) with x = 0 and y > 0

Fix _modpow() method in Math::BigInt::Calc so it correctly returns 0
in the case (0 ** $x) % $y, with $x > 0.

- lib/Math/BigInt/Calc.pm: fix mentioned bug

- t/bigintpm.inc: add a few more test cases

- t/bare_mbi.t: increment test counter

- t/bigintpm.t: increment test counter

- t/sub_mbi.t: increment test counter

5 files changed, 16 insertions, 8 deletions

diff --git a/dist/Math-BigInt/lib/Math/BigInt/Calc.pm b/dist/Math-BigInt/lib/Math/BigInt/Calc.pm
index a24400b166..387a231772 100644
--- a/dist/Math-BigInt/lib/Math/BigInt/Calc.pm
+++ b/dist/Math-BigInt/lib/Math/BigInt/Calc.pm
@@ -2383,11 +2383,17 @@ sub _modpow
     splice @$num,0,1; $num->[0] = 0;
     return $num;
     }
-  if ((scalar @$num == 1) && (($num->[0] == 0) || ($num->[0] == 1)))
-    {
-    $num->[0] = 1;
-    return $num;
-    }
+
+  # 0^a (mod m) = 0 if m != 0, a != 0
+  # 0^0 (mod m) = 1 if m != 0
+  if (_is_one($c, $num)) {
+      if (_is_zero($c, $exp)) {
+          @$num = 1;
+      } else {
+          @$num = 0;
+      }
+      return $num;
+  }
 
 #  $num = _mod($c,$num,$mod);	# this does not make it faster
 
diff --git a/dist/Math-BigInt/t/bare_mbi.t b/dist/Math-BigInt/t/bare_mbi.t
index 38b82afab3..e2186109f1 100644
--- a/dist/Math-BigInt/t/bare_mbi.t
+++ b/dist/Math-BigInt/t/bare_mbi.t
@@ -1,7 +1,7 @@
 #!/usr/bin/perl -w
 
 use strict;
-use Test::More tests => 3279;
+use Test::More tests => 3283;
 
 BEGIN { unshift @INC, 't'; }
 
diff --git a/dist/Math-BigInt/t/bigintpm.inc b/dist/Math-BigInt/t/bigintpm.inc
index 006ef4e618..530d0b52ee 100644
--- a/dist/Math-BigInt/t/bigintpm.inc
+++ b/dist/Math-BigInt/t/bigintpm.inc
@@ -1725,10 +1725,12 @@ abc:abc:5:NaN
 5:5:abc:NaN
 5:abc:5:NaN
 abc:5:5:NaN
+3:5:0:NaN
 # bmodpow Expected results
 0:0:2:1
 1:0:2:1
 0:0:1:0
+0:3:5:0
 8:7:5032:3840
 8:-1:5033:4404
 98436739867439843769485798542749827593285729587325:43698764986460981048259837659386739857456983759328457:6943857329857295827698367:3104744730915914415259518
diff --git a/dist/Math-BigInt/t/bigintpm.t b/dist/Math-BigInt/t/bigintpm.t
index 6e381e5529..f3a6da4426 100644
--- a/dist/Math-BigInt/t/bigintpm.t
+++ b/dist/Math-BigInt/t/bigintpm.t
@@ -1,7 +1,7 @@
 #!/usr/bin/perl -w
 
 use strict;
-use Test::More tests => 3279 + 6;
+use Test::More tests => 3283 + 6;
 
 use Math::BigInt lib => 'Calc';
 
diff --git a/dist/Math-BigInt/t/sub_mbi.t b/dist/Math-BigInt/t/sub_mbi.t
index c548cb3578..5c6f44cb03 100644
--- a/dist/Math-BigInt/t/sub_mbi.t
+++ b/dist/Math-BigInt/t/sub_mbi.t
@@ -1,7 +1,7 @@
 #!/usr/bin/perl -w
 
 use strict;
-use Test::More tests => 3279
+use Test::More tests => 3283
     + 5;	# +5 own tests
 
 BEGIN { unshift @INC, 't'; }